The present invention relates to a novel crystalline ion-association substance, a process of producing the same, and a photo-latent initiator for photopolymerization comprising the crystalline ion-association substance, which can be used in photopolymerization of cationically polymerizable organic material (the xe2x80x9cphoto-latent initiator for photopolymerizationxe2x80x9d of the present invention may be referred to xe2x80x9clatent photopolymerization initiatorxe2x80x9d).
Cationically polymerizable organic material, especially epoxy resin, is widely used as adhesives, sealants, paints, etc. in various fields such as automobile industry, housing/building material industry, civil engineering and construction industries, aircraft industry, and electric/electronic industry.
The cationically polymerizable organic material including epoxy resin is hardened (cured) by various means for polymerization to serve as adhesives, sealants, paints, etc. Photopolymerization is one of such means for polymerization, and many initiators for cationic photopolymerization have been well known.
The typical example of the initiators for photopolymerization of cyclic ether compound or ethylene unsaturated compound is an onium salt which contains an element having a lone pair to which either a proton or other cationic compound is bonded with a coordinate bond. Particular examples of such onium salts are aromatic diazonium salts, aromatic iodonium salts, and aromatic sulfonium salts. Many of the onium salts have halogen metal complex anions (BF4xe2x88x92, PF6xe2x88x92, AsF6xe2x88x92, SbF6xe2x88x92, etc.) as counter ions.
The working mechanism of these conventional initiators for photopolymerization such as diazonium salt, iodonium salt, and sulfonium salt are shown in the following Scheme I, II, III, respectively. In any of these cases, Bronsted acid is generated at first by light irradiation. 
The generated Brxc3x8nsted acid reacts with the cationically polymeriz able organic material and the polymerization proceeds in accordance with the following Scheme IV, whereby the polymer chain grows. 
Another example is a salt of metallocene complex. In this case, Lewis acid is produced as active species by light irradiation, and an insertion of monomer occurs at the Lewis acid, whereby the polymer chain grows in accordance with the following Scheme V. 
Also, the initiator for photopolymerization that comprises borate counter anion has been disclosed in Japanese Patent Laid-open Nos. 143,044/87 and 182,701/90. These literatures disclose the working mechanism of the initiator for photopolymerization, which is shown in the following Scheme VI. In accordance with the scheme, dyestuff which is the cation component of the complex is excited by light irradiation to the singlet state; the dyestuff of singlet state receives electron from a borate salt which is the anionic component; then the generated borate radical dissociates one of the ligands to generate a radical; whereby a radical polymerization proceeds.
BR4xe2x88x92D+xe2x86x92D.+B R4.
BR4.xe2x86x92B R3+R.
D: cation dyestuff or transition metal complex cation
B: boron atom
R: ligand
Also, Masahiro TSUNOOKA described the anionic polymerization in which a base is generated by light irradiation (photo-initiating anionic polymerization) in Polymers, Vol.45, November, 786-789 (1996), but this study has just started.
In this reaction system, a problem of metal corrosion by acid, which has seen in the case of using the conventional acid-generating type initiator, does not occur; however, since the photo-initiating anionic polymerization is a consecutive reaction with respect to the base produced by light irradiation, the chain-growth reaction is difficult in comparison with the chain-growth reaction in the cationic polymerization by acid. In addition, outgassing of carbonic acid gas, and so on occurs by light irradiation, as well as production of base. Therefore, it includes some problems to be solved.
A typical example of the base-generating type initiators is shown in the following Scheme VII. 
In the case of using the conventional initiator for photopolymerization in form of a homogeneous dispersion or a mixture with the cationically polymerizable material, it was impossible to completely harden the polymerizable material only by light irradiation.
Thus, since the conventional initiator for photopolymerization generates at first either Bronsted or Lewis acid or base as active species by light irradiation, the following subjects to be solved are found.
1) It is impossible to harden thick film unless using large amount of initiator or using jointly light irradiation and heating.
2) The storage stability of the composition comprising the initiator and the polymerizable organic material is poor.
3) The electrolytic corrosion to metal is strong.
4) In the production of printed circuit boards and mounting, it is impossible to accomplish fine patterning and high package density, because the conventional initiator has high dependency on heat in polymerization.
5) The produced polymer (hardening material) is deficient in resistance to moisture (water).
6) The produced polymer (hardening material) is deficient in adhesiveness to substrate.
7) The application to substrate that is apt to deform by heat is limited.
8) The inorganic halogen compound anion that is present in many of the initiators decreases diffusion properties such as affinity and compatibility between the initiator and the various components when mixed with the polymerizable material.
9) Since the photopolymerization using the base-generating type initiator (anionic polymerization type) is a consecutive reaction, it is difficult to establish the chain-growth reaction in comparison with the cationic polymerization using the acid-generating type initiator.
10) Since the polymerization (hardening) of epoxy resin by base is a stoichiometrical reaction, it is necessary to use large amount of base-generating type initiator.
11) Since the polymerization of epoxy resin by base is highly dependent on heat, a post-treatment of hardening by heat is required.
12) The base-generating type initiator produces not only the base, but also carbonic acid gas by light irradiation. Therefore, this outgassing constitutes one of the problems to be solved.
The inventor has earnestly tried to solve the problems of the above-mentioned conventional initiator for photopolymerization, and has found a novel photo-latent initiator for photopolymerization, which has an ability to polymerize the cationically polymerizable organic material only with light irradiation. Also, the photo-latent initiator for photopolymerization of the present invention can display a stability in any case of existing alone or in the form of a mixture with cationically polymerizable organic material and possible other additives, and does not lose the activity as initiator for photopolymerization even after long-term preservation.
According to the present invention, the photo-latent initiator for photopolymerization comprises a crystalline ion-association substance having a general formula (I):
[{C5(R1)n}2mMm]1+[{B(R2)4}xe2x88x92]l
wherein M is a transition metal of center nucleus; C5 is cyclopentadienyl group; R1 is electron-donating substituent bonded to carbon atom of cyclopentadienyl group; n is either 4 or 5; m is either 1 or 2; l is either 1 or 2; R2 is ligand coordinated to boron atom (B), and four (R2)s are the same.
The crystalline ion-association substance that constitutes the photo-latent initiator for photopolymerization of the present invention is a novel substance, which constitutes one of objects of the present invention.
Thus, an object of the present invention is a novel crystalline ion-association substance having a general formula (I):
[{C5(R1)n}2mMm]1+[{B(R2)4}xe2x88x92]l
wherein M is a transition metal of center nucleus; C5 is cyclopentadienyl group; R1 is electron-donating substituent bonded to carbon atom of cyclopentadienyl group; n is either 4 or 5; m is either 1 or 2; l is either 1 or 2; R2 is ligand coordinated to boron atom (B), and four (R2)s are the same.
Another object of the present invention is a photo-latent initiator for photopolymerization of cationically polymerizable organic material, characterized in that said photo-latent initiator for photopolymerization comprises the crystalline ion-association substance having the general formula (I):
[{C5(R1)n}2mMm]1+[{B(R2)4}xe2x88x92]l
wherein M is a transition metal of center nucleus; C5 is cyclopentadienyl group; R1 is electron-donating substituent bonded to carbon atom of cyclopentadienyl group; n is either 4 or 5; m is either 1 or 2; l is either 1 or 2; R2 is ligand coordinated to boron atom (B), and four (R2)s are the same.
Also, another object of the present invention is a process of producing the novel crystalline ion-association substance having the above-mentioned formula (I), characterized in that a metallocene derivative of either mono-nucleus or di-nucleus structure having a general formula (II)
{C5(R1)n}2mMm
wherein M, C5, R1, m, and n have the same meaning as mentioned above, is reacted with a tetradentate borate complex compound having a general formula (III)
{B(R2)4}X
wherein R2 has the same meaning as mentioned above, and X is an alkali metal atom.